1. Technical Field
The present invention relates to a configuration of circuit components in an integrated circuit (IC) to enable use of the IC to measure pulse width relative to the frequency of the system clock in the IC. More particularly, the present invention relates to measuring time delay from transmission until receipt of a signal.
2. Related Art
A simple method to measure pulse width in an IC is to use a free running counter connected to the IC system clock. Circuitry is connected to the counter that detects the beginning transition of the pulse to start the counter, and then to detect the next transition of the pulse to stop the counter. The count then gives an accurate measurement of the pulse width.
A problem using a counter and system clock to measure a pulse width occurs when the delay is small and approaches the limits of the system clock frequency. In a typical IC, such as a Field Programmable Gate Array (FPGA), it is difficult to have a clock speed exceeding 1 GHz, leading to measurement resolution accuracy limits when pulses are on the order of 10 nanosecond or less.
A number of systems require measurement of pulses less than 10 ns. An example of such a system is a collision avoidance system for automobiles. Measurement of pulses to the order of 100 picoseconds enables measuring the distance to close-by objects with a resolution on the order of centimeters. A typical collision avoidance system can measure small distances on the order of centimeters to keep a safe distance from cars that a host vehicle is following in traffic.
Two different types of collision avoidance sensing systems are typically used as illustrated in U.S. Pat. No. 6,590,495 entitled “Automobile Distance Warning and Alarm System.” A first type system uses a rangefinder that includes a laser pulsed-echo sensor. The laser operates at a high frequency and can generate short pulses enabling measurement of short distances easily, such as when following another automobile in traffic. The laser sensor is also relatively compact and can be concealed, such as in the grill area of a car. The laser device has some deficiencies such as penetrating fog or operating over longer distances. A second type system is a radar system that uses lower frequency radar components to form a rangefinder. The lower frequency radar signals still typically operate with a frequency range on the order of 1 GHz and higher, but work well over a longer distance and better through fog than laser rangefinders. The lower frequency radar antennas, however, typically require large horn antennas that are not easily mounted or concealed in a vehicle. With a limit on system clock frequencies in ICs, a standard IC chip like an FPGA used to measure distance will only marginally determine distances for a radar system, and will likely be unusable for some pulse-laser systems that require measurement resolution of better than 10 ns.
FIG. 1 illustrates components of a typical automobile distance warning system that uses a pulsed-laser range finder for distance measurement. The components include a range finder 102, a time measuring circuit 104 and a microcontroller 106. The range finder 102 illustrated is a laser pulse-echo device that can be mounted on the front of a car to detect vehicles or objects to be avoided. The time measuring circuit 104 includes a transmitter 114 that sends signals to cause the pulsed infrared laser diode 110 to send out pulses. The infrared photo transistor 112 receives reflected pulses and provides them to receiver 116. Illustratively, the transmitter 114 operates at a pulse repetition rate of 10 MHz so that one pulse is emitted every 100 nanoseconds.
With the transmitted and reflected laser pulses provided, a digital clock 117 and a counter 118 are used to determine the time interval between the initiation of the transmitted pulse and the return of the reflected pulse. In particular, a signal from a transmitter 114 in time measurement circuit 104 causes counter 118 to begin counting clock pulses when an infrared pulse is emitted by the laser diode 110; and a signal from receiver 116 causes counter 118 to stop counting when the reflected infrared pulse is received by receiver 116. The count in counter 118 is proportional to the separation distance from the host vehicle to the detected object. Time measuring circuit 104 provides the count from counter 118 to microcontroller for calculation of the separation distance from the host vehicle to the object detected. The microcontroller 106 then functions to control the vehicle speed or alert the operator of a possible collision.
With collision avoidance systems, as well as other systems that require pulse width measurement, it may be desirable to measure pulse separations of 10 ns or less. It is thus desirable to provide a system in an IC having a maximum clock speed that can provide for measurement of pulse separations less than measurable directly from the available clock count in the IC.